The invention relates to a method for operating an internal combustion engine. The invention further relates to an internal combustion engine.
The internal combustion engine is used, for example, to drive a motor vehicle, thus providing a torque directed to a drive of the motor vehicle. The internal combustion engine is charged and has the exhaust gas turbocharger for this purpose. The exhaust gas turbocharger has a turbine and a compressor operatively connected to the turbine. The exhaust gas generated by the internal combustion engine during operation is supplied to the turbine, with the turbine converting the enthalpy contained in the exhaust gas and/or the flow energy (especially rotational movement) into kinetic energy. The kinetic energy is used to drive the compressor.
The compressor is used to condense air taken in, in particular fresh air, i.e. to bring it to a higher pressure level. The compressed air is then supplied to the internal combustion engine where it contributes to the performance of the combustion. By increasing the pressure level prior to supplying the air into the internal combustion engine, the amount of air available during combustion can be increased so that the power of the internal combustion engine can be significantly increased.
Furthermore, the internal combustion engine has the at least one catalyst, which is used to clean the exhaust gases generated by the internal combustion engine. The exhaust gases are thus passed through the catalyst and only then discharged into the surroundings of the internal combustion engine. Preferably, the catalyst is arranged downstream of the exhaust gas turbocharger so that the exhaust gas of the internal combustion engine first flows through the exhaust gas turbocharger and only then the catalyst.
Finally, the internal combustion engine has the variable valve train which enables the influence of valve timing of at least one intake valve and/or an exhaust valve of the internal combustion engine. Preferably, the internal combustion engine has a plurality of cylinders, wherein each of the cylinders are assigned at least one intake valve and one exhaust valve. The intake valve and the exhaust valve or rather all intake valves and all exhaust valves of the internal combustion engine are associated with the valve train. The variable design of the valve train now makes it possible to specifically influence or set the valve timing of the intake valves, exhaust valves or rather both intake valves and exhaust valves.
It is provided in the at least one operating mode that the variable valve train is set such that a valve overlap exists. By this, it is understood that the at least one intake valve and the at least one exhaust valve of this cylinder are simultaneously opened during the exchange of gases in the cylinder. Air streaming in through the intake valve in the cylinder can thus immediately flow out again through the exhaust valve. If there is a positive pressure difference between the intake side of flow before the intake valve and the exhaust side of flow after the exhaust valve, then the cylinder is flushed with air, in particular fresh air, wherein a part of the air takes along the hot exhaust gas found in the cylinder to the exhaust side, in particular in an exhaust manifold of the internal combustion engine.
The filling is significantly increased by such a measure compared to a conventional operation in which there is no valve overlap. Because an increased mass flow on the exhaust side is simultaneously present, the response behavior of the exhaust gas turbocharger is improved, particularly at low rotational speeds of the internal combustion engine. In addition, the amount of exhaust gas remaining in the cylinder, which is also referred to as a residual gas proportion, is reduced, causing a reduction of the combustion chamber temperature. This leads to a reduced tendency to knock, particularly at full load of the internal combustion engine. The valve overlap is determined by the valve overlap parameters. Thus, the greater the valve overlap parameter selected, the greater the valve overlap. In contrast, there is no valve overlap for a valve overlap parameter of zero. The valve overlap parameter is now determined in the at least one operating mode and set on the valve train so that the internal combustion engine is operated with the appropriate valve overlap.
A disadvantage of the valve overlap, however, is that the increased amount of air, which corresponds to a larger amount of oxygen, can lead to saturation of the oxygen storage capability of the catalyst and thus to a drop in the NOx conversion performance. Accordingly, there is a conflict between the advantages described above and the disadvantage that the emission limit values, particularly for NOx, may be impaired.
One possible remedy is that the internal combustion engine is operated with a richer fuel-air mixture. This leads, through after-burning, to an improvement or rather a reduction of emissions, however, simultaneously causing a higher thermal loading of the catalyst. In addition, the carbon dioxide emissions is increased by the increased use of fuel. Particulate emissions may also worsen.
For example, the publications DE 10 2012 204 885 B3 and GB 2 367 859 A are known from the prior art. The former describes a method for operating an internal combustion engine with at least one cylinder which has an exhaust gas discharge system for discharging the exhaust gases and at least one exhaust gas after-treatment system arranged in this exhaust gas discharge system. A first lambda sensor for detecting a first air ratio is arranged upstream of the at least one exhaust after-treatment system and a second lambda sensor for detecting a second air ratio is arranged downstream of the at least one exhaust gas treatment system. Depending on an operation of the internal combustion engine, either the first lambda sensor or the second lambda sensor should now be used by means of engine control for the control of the air ratio, wherein at least one engine characteristic map stored in the engine management system using at least two operating parameters of the internal combustion engine specifies which lambda sensor is to be used for control of the air ratio as input variables.
Publication DE 103 03 705 A1 is further known from the prior art. This describes a method for operating an internal combustion engine (which has a variable valve train) working with direct fuel injection and provided with a charging device. The valve overlap of the gas exchange valves is thereby set for a full load operation or at least near full load operation of the internal combustion engine by adjusting the gas exchange valve timing on stationary set points. This performs a dynamic correction of the stationary set points toward higher valve overlap values after gas exchange cycle TDC with an elevated load requirement in the charged operation. The dynamic correction values are weighted by means of a first factor which is a function of the pressure difference between the pressure set point and the actual pressure in the intake passage and is weighted by means of a second factor which takes into account the temperature of the exhaust gas catalyst.
In addition, the publication AT 501 678 A2 describes a method for operating an external ignition internal combustion engine with direct fuel injection, which has at least one exhaust gas turbocharger and at least one exhaust gas treatment device, with at least one device for altering the intake and/or exhaust timing, wherein the internal combustion engine, in particular at low rotational speed, is operated with a defined valve overlap of intake and exhaust valves for flushing the combustion chamber, and wherein the valve overlap is changed as a function of at least one engine operating parameter. In order to improve the torque of the internal combustion engine, particularly at low engine rotational speed, it is provided that the flushing is limited or reduced after reaching a threshold value of the engine operating parameter.
Finally, publication DE 10 2006 025 050 B4 shows a method and device for operating an exhaust gas after-treatment system and the publication DE 10 2011 005 516 A1 shows a method and a device for detecting at least one characteristic of a gas.